1. Field of the Invention
The present invention relates generally to an apparatus and method for generating and receiving traffic in a code division multiple access mobile communication system, and more particularly to an apparatus and method for generating and receiving traffic in a code division multiple access mobile communication system using a block spreading scheme.
2. Description of the Related Art
Conventionally, a code division multiple access (CDMA) mobile communication system has been well known as a representative example of a wireless communication system. Many methods using the CDMA mobile communication system have been proposed from a DS (Direct Sequence)-CDMA, through many developments, to a BS (Block Spreading) CDMA. A multiple access interference (MAI) is always considered in these methods. The MAI is generated by an inter-signal random time offset in the CDMA mobile communication system where users have different individual codes. The MAI can be ignored if the CDMA mobile communication system has only one user or a few users, but becomes large when the system has many users.
Further, mobile communication systems have gradually developed from systems providing only voice services to systems providing simple data services. For example, a system called 1x E-DO for transmitting high speed packet data is used. In addition, standardization of a 1x E-DV, which is capable of transmitting voice and high speed packet data together, has currently come to the finish.
As the high speed packet data transfer is achieved, the MAI has a significant effect on the data transfer. Accordingly, many studies have been made to reduce the MAI. One of these systems is the DS-CDMA system. FIG. 1 is a block diagram of signals of spread of data to be transmitted in the DS-CDMA system.
Referring to FIG. 1, reference numeral 100 designates a first data of a particular user. Additionally, in FIG. 1, (k) in si(k) is a k-th user and i is i-th data. In addition, (k) in cp(k) is a k-th user and p is i-th spread code. Reference numerals 101, 102, 103, 110, 111 and 113 identify data resulting from spreading each data symbol by a particular spread code. The spread data is transmitted from a base station to a mobile terminal via a multi-path during forward transmission and is transmitted from the mobile terminal to the base station via the multi-path during reverse transmission.
FIG. 2 is a timing diagram illustrating the transmission of the spread data via the multi-path. It is assumed in FIG. 2 that a delay is generated during the transmission of data via a forward link. Also, it is assumed that start times of transmission of data to be transmitted to different users are synchronized. A spread data stream 210 is a data stream received via a straight line of a first user. The same data as illustrated in FIG. 1 is received at a start time designated by the reference numeral 200. A data stream 220 is the same data stream as the data stream illustrated in FIG. 1, but has one chip delay and is received via a path other than the straight path. Also, a data stream 230 is the same data as the data stream received via the straight path, but has a two chip delay and is also received via a path other than the straight path.
Data stream 240 is received via a straight path of a different user. Data streams received via the same path are data streams spread by different orthogonal spreading codes. Therefore, for example, when a chip 211 and a chip 241, which are simultaneously received in a receiver, i.e., the mobile terminal, via the same path, are despread in the receiver, the chip 241 is removed by its orthogonality with the chip 211. However, a first chip 221 of the data stream 220 having a delay of one chip interferes with a second chip 242 of the data stream 240 from the different user because the delayed chip 221 has a low probability of maintaining orthogonality with the second chip, which results in deterioration of quality of the received signal. The same is true to the relation between chip 231 of the data stream 230 and chip 243 of the data stream 240.
Although the receiver can decode delayed and spread data streams received through respective fingers via multiple paths, it is possible that an interference of a data stream of a user with a different data stream of a different user may occur. Although one path of the different user is illustrated in FIG. 2, there may be other paths of the different users to cause interference. Therefore, the greater the number of users and paths is, the greater the interference is.
FIG. 3 illustrates interference simulation results when the number of users (K>=1) and the numbers of paths are 1, 3, and 5. In FIG. 3, reference numeral 301 designates a band of a signal-to-noise ratio required for voice traffic and reference numeral 302 designates a band of a signal-to-noise ratio required for data traffic. In addition, reference numeral 310 designates a simulation result for the signal-to-noise ratio and a bit error rate (BER) in one path, i.e., a straight path, reference numeral 320 designates a simulation result for the signal-to-noise ratio and the BER in three paths, and reference numeral 330 designates a simulation result for the signal-to-noise ratio and the BER in five paths. As can be seen in FIG. 3, when three and five paths are used, the BER converges in a band of 1.00E-02. In other words, as the number of paths increases, it becomes difficult to improve the BER.
The BS-CDMA system has been proposed to overcome the problem occurring when the DS-CDMA system is used. FIG. 4 is a diagram illustrating a spread data stream received by a particular user in the mobile communication system using the BS-CDMA system.
Referring FIG. 4, each of reference numerals 410, 420 and 430 represents a spread of a plurality of data streams by one spreading code. Chip 1 410, chip 2 420, and chip p 430 each consist of, for example, 100 symbols. Data symbols included in the same chip are spread by the same spreading code. User symbols S1(k),S2(k), . . . , SM(k) included in chip 1 410 are spread by a spreading code c1(k). User symbols S1(k),S2(k), . . . , SM(k) included in chip 2 420 to be transmitted next are spread by a next spreading code c2(k). Therefore, in the BS-CDMA system, spreading codes are changed in a certain unit, which enables the MAI by the multiple paths to be reduced.
FIG. 5 is a timing diagram of spread data streams of two users received via multiple paths in the BS-CDMA system. Herein below, a procedure of receiving data streams of two users via the multiple paths and a resultant MAI in the BS-CDMA system will be described in detail with reference to FIG. 5.
Referring FIG. 5, reference numerals 510, 520, and 530 indicate spread data streams of a first user received via respective different paths. Chip 1, chip 2, and chip p in the data stream 510 are intended to represent chip units. The spread data streams are received in the chip units, as illustrated in FIG. 4. It is assumed in FIG. 5 that a delay is generated in one symbol unit. When the delay is generated in one symbol unit, the data stream 510 is received with the delay, as shown by data streams 520 and 530.
However, assuming that spread data streams of a different user are received via the same path in synchronization with the data streams of the first user, the data stream 540 of the different user is received via the same path as the data stream 510. In addition, the data stream 550 of the different user is received via the same path as the data stream 520 and the data stream 560 of the different user is received via the same path as the data stream 530. In such a receipt, there are intervals 501, 502, . . . , 503, as illustrated in FIG. 5, where interference due to a delay difference between the data streams of the first user and the data stream of the different user occurs. In these intervals, because there is no data interference, the data streams can be more precisely received without considering the interference by the MAI.
Data streams to be transmitted from the base station to the user are segmented into predetermined blocks. Symbols in each of the blocks are spread by a code allocated for each block. Accordingly, a signal to be transmitted to a k-th user is expressed as in Equation 1,
                                          X            k                    ⁡                      (            t            )                          =                              ∑                          j              =              1                        M                    ⁢                                    s              j                              (                k                )                                      ⁢                                          ∑                                  i                  =                  1                                P                            ⁢                                                c                  i                                      (                    k                    )                                                  ⁢                                  g                  ⁡                                      (                                          t                      -                                              iMT                        s                                            -                                              jT                        s                                                              )                                                                                                          Equation        ⁢                                  ⁢        1            where k is the index of each user, sj(k) is a transmit symbol at jth timer, and ci(k) is ith bit of the spreading sequence.
In Equation 1, g(u) is determined by Equation 2.
                              g          ⁡                      (            u            )                          =                  {                                                    1                                                              0                  ≤                  u                  <                                      T                    s                                                                                                      0                                                                                  u                    <                    0                                    ,                                      u                    ≥                                          T                      s                                                                                                                              Equation        ⁢                                  ⁢        2            
In Equation 2, Ts indicates a duration during which one symbol in one chip exists in a slot. Equation 1 is well known as an equation for data streams as shown in FIG. 4 in the case of the DS-CDMA system, and therefore, a detailed explanation thereof will be omitted for the purpose of brevity.
However, normalized orthogonal codes such as Walsh codes are codes used for different users. Therefore, even if the base station transmits traffic data to a plurality of users, traffic sequences received by a particular user are deleted by the orthogonal Walsh codes, as shown in Equation 3.
                                          ∑                          i              =              1                        P                    ⁢                                    c              i              k                        ⁢                          c              i                              k                ′                                                    =                  {                                                    1                                                              k                  =                                      k                    ′                                                                                                      0                                                              k                  ≠                                      k                    ′                                                                                                          Equation        ⁢                                  ⁢        3            
In Equation 3, k represents a particular user using a particular Walsh code. Therefore, a plurality of user signals transmitted from the base station can be expressed as in Equation 4.
                              S          ⁡                      (            t            )                          =                                            ∑                              k                =                1                            K                        ⁢                                          X                k                            ⁡                              (                t                )                                              =                                    ∑                              k                =                1                            K                        ⁢                                          ∑                                  j                  =                  1                                M                            ⁢                                                s                  j                                      (                    k                    )                                                  ⁢                                                      ∑                                          i                      =                      1                                        P                                    ⁢                                                            c                      i                                              (                        k                        )                                                              ⁢                                          g                      ⁡                                              (                                                  t                          -                                                      iMT                            s                                                    -                                                      jT                            s                                                                          )                                                                                                                                                    Equation        ⁢                                  ⁢        4            
In Equation 4, it should be assumed that the number (k) of users is smaller than the number (P) of paths.
In addition, assuming that a channel of the forward link is a frequency selection channel with the maximum delay time LTs, a receipt signal of a particular k-th user can be expressed as in Equation 5.
                              r          ⁡                      (            t            )                          =                                            ∑                              l                =                0                                            L                -                1                                      ⁢                                          h                l                            ⁢                                                ∑                                      k                    =                    1                                    K                                ⁢                                                      ∑                                          j                      =                      1                                        M                                    ⁢                                                            s                      j                                              (                        k                        )                                                              ⁢                                                                  ∑                                                  i                          =                          1                                                P                                            ⁢                                                                        c                          i                                                      (                            k                            )                                                                          ⁢                                                  g                          ⁡                                                      (                                                          t                              -                                                              iMT                                s                                                            -                                                              lT                                s                                                                                      )                                                                                                                                                                            +                      n            ⁡                          (              t              )                                                          Equation        ⁢                                  ⁢        5            
In Equation 5, h1 indicates a complicated fading factor of an 1-th delay path and n(t) is additive white Gaussian noise (AWGN). When designing a receiver, in Equation 5, M should be larger than L. M is the number of gap symbols in a frame and L is the index of each multipath signal.
Accordingly, a receiver (i.e., the mobile terminal) receiving the data streams performs a despread process for data detection. This despread process is performed according to the above Equation 5. Accordingly, an output of the despreading for a j-th symbol time slot in one block can be expressed as in Equation 6.
                              y          j                      (                          k              ′                        )                          =                                            ∑                              i                =                1                            P                        ⁢                                          c                i                                  (                                      k                    ′                                    )                                            ⁢                              r                ⁡                                  (                                                                                    i                        ′                                            ⁢                                              MT                        s                                                              +                                                                  j                        ′                                            ⁢                                              T                        s                                                                              )                                                              ⁢                                          ⁢                                          =                                                    ∑                                  l                  =                  0                                                  L                  -                  1                                            ⁢                                                h                  l                                ⁢                                                      ∑                                          k                      =                      1                                        K                                    ⁢                                                            ∑                                              j                        =                        1                                            M                                        ⁢                                                                  s                        j                                                  (                          k                          )                                                                    ⁢                                                                        ∑                                                                                    i                              ′                                                        =                            1                                                    P                                                ⁢                                                                              ∑                                                          i                              =                              1                                                        P                                                    ⁢                                                                                    c                              i                                                              (                                k                                )                                                                                      ⁢                                                          c                              i                                                              (                                                                  k                                  ′                                                                )                                                                                      ⁢                                                          g                              ⁡                                                              (                                                                                                                                            i                                      ′                                                                        ⁢                                                                          MT                                      s                                                                                                        -                                                                                                                                          ⁢                                                                                                                                          ⁢                                                                      iMT                                    s                                                                    +                                                                                                            j                                      ′                                                                        ⁢                                                                          T                                      s                                                                                                        -                                                                      jT                                    s                                                                    -                                                                      lT                                    s                                                                                                  )                                                                                                                                                                                                                              +                                          ∑                                                      i                    ′                                    =                  1                                P                            ⁢                                                c                                      i                    ′                                                        (                                          k                      ′                                        )                                                  ⁢                                  n                  ⁡                                      (                                                                                            i                          ′                                                ⁢                                                  MT                          s                                                                    +                                                                        j                          ′                                                ⁢                                                  T                          s                                                                                      )                                                                                                          Equation        ⁢                                  ⁢        6            In addition to the definition on the line for equation 1, hl is the impulse response of lth multipath.
Then, by substituting Equation 3 into Equation 6, despread outputs of an M-th symbol to an L-th symbol can be expressed as the following Equation 7.
                                          y            j                          (                              k                ′                            )                                =                                                    ∑                                  l                  =                  0                                                  L                  -                  1                                            ⁢                                                h                  l                                ⁢                                  s                                      j                    -                    l                                                        (                                          k                      ′                                        )                                                                        +                                          ∑                                                      i                    ′                                    =                  1                                P                            ⁢                                                c                                      i                    ′                                                        (                                          k                      ′                                        )                                                  ⁢                                  n                  ⁡                                      (                                                                                            i                          ′                                                ⁢                                                  MT                          s                                                                    +                                                                        j                          ′                                                ⁢                                                  T                          s                                                                                      )                                                                                      ⁢                                  ⁢                  where          ,                      j            =            L                    ,          …          ⁢                                          ,          M                                    Equation        ⁢                                  ⁢        7            
As can be seen from the above Equation 7, the MAI for first L−1 symbols is still present as ISI (Inter Symbol Interference), which has an effect on detection of M−L+1 symbols. This can be verified from FIG. 5. Namely, there is interference due to the MAI between front symbols and rear symbols in an interval between the reference numeral 501 and the reference numeral 502.
However, as described above in connection with FIG. 5, the interval where the MAI is not generated is not a continuous interval. Therefore, there is an interval where interference occurs. Accordingly, a receiver should be configured such that it processes an interval with the interference and an interval without the interference separately. However, in this case, the receiver may become complex. Accordingly, methods have been suggested to overcome this complexity problem at the time of transmission of data streams. These methods will be hereinafter considered.
FIG. 6A is a diagram illustrating a method for constructing a data stream in order to reduce interference caused by the MAI in the BS-CDMA system, and FIG. 6B is a diagram illustrating another method for constructing a data stream in order to reduce interference caused by the MAI in the BS-CDMA system.
Referring to FIG. 6A when a data stream of the BS-CDMA system is constructed using a cyclic structure, symbols in which the MAI may occur, in a last part of a traffic to be transmitted, are copied, and the copied symbols are transmitted in advance. More specifically, symbols 601 are copied, and the copied symbols are attached ahead of symbol data 602 to be transmitted together. The method illustrated in FIG. 6A enables more efficient reproduction of data at a data receipt side.
Referring to FIG. 6B when the data stream is constructed by using a zero-padding as a non-cyclic structure, the predetermined number of “0”s are inserted into the data stream to be transmitted in order to reduce interference of a last part in the data stream. More specifically, the predetermined number of “0”s are inserted into the data stream during a predetermined interval designated by the reference numeral 603, which results in ease of data transmission as well as performance improvement of the receiver.
However, the methods of FIGS. 6A and 6B have a number of problems. In the method illustrated in FIG. 6A, because symbols are copied and displaced beforehand, combination of symbols should be carried out with displaced symbols and non-displaced symbols separated. In addition, because the MAI of the displaced symbols has an effect on front and rear symbols in the data stream, identical symbols should be simultaneously processed in order to increase the efficiency of reduction of interference between symbols. This increases the complexity of the circuits of the receiver.
In the method illustrated in FIG. 6B, the insertion of “0”s into the end of the data stream causes a problem of deterioration of bandwidth efficiency of a system. In addition, it is difficult for the system to transmit symbols “0”s. Consequently, the complexity of the system may be increased.